1. Field of the Invention
This invention relates to an optoelectronic field and, more specifically, to a visible laser light source and laser applied units employing a visible laser light source, such as a laser printer, fine particle detector, optical shaping unit, optical recorder and the like.
2. Related Art
With the progress of the advanced information communication age, demand for adaptation to short wavelengths is arising to meet requirements for improved recording density and high-speed printing in an optical recording field such as optical disk drives and laser printers. However, as light sources capable of providing a blue color range (wavelength of 400 to 480 nm) which is in high demand for commercialization, only gas lasers such as He--Cd (helium-cadmium) and Ar (argon) lasers are available, which have been unsuitable for use in optical disk drives, for example, because they are bulky and power consuming. Although the above gas lasers are actually incorporated in some laser printers as a light source, there is the possibility that they will be an obstacle to future reductions in the size and power consumption of laser printers for the above reason.
To overcome the above problem, technology which makes use of second harmonic generation (to be abbreviated as "SHG" hereinafter) to reduce wavelength is proposed. Progress has been made in studies on technology for practical application of this SHG light source along with an increase in the output of a semiconductor laser. The scene behind this is that discharge is not necessary for this SHG light source unlike conventional gas lasers, there is the possibility that (1) the size and (2) power consumption of the SHG light source will be reduced, and the SHG light source has high reliability depending on the output stability and long service life of an excitation semiconductor laser ((3) output stability and (4) long service life).
There is also proposed a method for obtaining blue radiation 12 as a second oscillation wave, that is, an SHG wave, from an SHG light source having the same output wavelength as that of the afore-mentioned gas laser, for example, wherein the output of a semiconductor laser 1 generating near infrared light as shown in FIG. 10 is taken as a first oscillation wave, that is, a fundamental wave, and resonated in a monolithic external resonator 44 which is composed of a nonlinear crystal (W. J. Kozlovsky and W. Lenth, "Generation of 41 mW of blue radiation by frequency doubling of a GaAlAs diode laser", Appl. Phys. Lett., Vol.56 No.23, p.2291, 1990). The nonlinear optical crystal (to be referred to as "SHG crystal" because wavelength conversions hereinafter are all for SHG) is KN (KNbO.sub.3 : potassium niobate).
However, the above SHG light source involves the following advanced technical problems. One of them is that it is necessary to adjust the oscillation wavelength of the semiconductor laser 1 which is easily affected by disturbance to a wavelength at which the SHG conversion efficiency of a KN crystal becomes maximal. For this purpose, an optical isolator 42 must be inserted to protect the semiconductor laser from light reflected by the KN crystal. Another problem is that reflected light from the resonator must be received by an optical detector 45 to control the length of an external resonator including the KN crystal in the order of the wavelength of the fundamental wave and the electrical output of the optical detector 45 is supplied to a feedback circuit 46 to control the output of the semiconductor laser for stable oscillation. Therefore, it is expected to be difficult to find solutions to these technical problems for commercialization.
Means for solving the above technical problems include an intracavity doubling SHG laser system in which an oscillation wave from a solid laser is taken as a fundamental wave and an SHG crystal is arranged in the resonator of the solid laser. In other words, in the intracavity doubling SHG laser system, oscillation wavelength is rarely affected by disturbance such as the above reflected light because the resonator forming the solid laser comprises mirrors having a high reflectance for the wavelength of an oscillation wave from the solid laser arranged at both ends thereof. The intracavity doubling SHG laser system is further characterized in that SHG conversion efficiency is hardly affected by fluctuations in oscillation wavelength caused by changes in the length of a resonator in the order of wavelength resulted by temperature variations and vibrations, unlike an external resonator SHG system.
A laser using a LiSAF (Cr:LiSrAlF.sub.6 ; chromium added lithium strontium aluminum fluoride) crystal as a laser crystal which oscillates at a wavelength of 750 to 1,000 nm has recently been proposed as a semiconductor laser excited wavelength variable solid laser (U.S. Pat. No. 4,811,349).
The inventors of the present invention have studied a method for causing a nonlinear crystal to generate SHG light having a blue color range as a second oscillation wave, using laser light from this semiconductor laser excited LiSAF crystal as a first oscillation wave (fundamental wave), and found two new problems involved.
FIG. 9 is a structural diagram for second harmonic generation by a LiSAF crystal and a nonlinear crystal. A first laser mirror 3 formed of a dielectric multi-layer film reflecting 99% or more of a fundamental wave oscillated from the LiSAF crystal 4 and transmitting excitation light is formed on the surface of the LiSAF crystal 4 where excitation light 11 from a semiconductor laser (unshown) is input and a laser resonator is formed between the first laser mirror and a second laser mirror 7 as a curvature mirror arranged on the output side thereof. The resonator has therein an SHG crystal 6 and a birefringent crystal 5 which is an element for controlling the wavelength of the fundamental wave and SHG light is output from the second laser mirror 7. The second laser mirror 7 is coated with a coating to reflect 99% or more of the fundamental wave and transmit SHG light.
The first problem in the structure of FIG. 9 is that it is impossible to generate SHG light efficiently because the beam waist of a resonator beam 32 of the generated fundamental wave is located at the first laser mirror 3 on the LiSAF crystal 4 and is large in diameter at the 15 nonlinear crystal 6 for generating SHG light. This is because SHG conversion efficiency generally depends on the beam diameter of the fundamental wave within the nonlinear crystal and the smaller the beam diameter, the more efficiently the SHG light can be generated.
The second problem is that part of SHG light 31 is reflected by the birefringent crystal which is a wavelength control element 5 because the SHG light generated by the nonlinear crystal crosses the polarization direction of the fundamental wave at a right angle. This is because the birefringent crystal transmits the fundamental wave because it is inclined at the Brewster angle with respect to the polarization of the fundamental wave, but has a low transmittance for the polarization of SHG light.